1. Field of the Invention
Solid-state medium-voltage power converters utilize semiconductor devices to convert a three-phase incoming power to an adjustable medium-voltage output. The outputs can be from multiphase AC or variable-voltage DC. Frequency or voltage control of the output is delivered to a load. The load can be either AC or DC, and, in certain drive applications, may be a multiphase AC motor. Because most solid-state converters use switching to obtain variable frequency and/or variable voltage outputs, harmonic considerations, both in the power line that feeds the drive and in the load, are of a high concern. It is desirable to limit the harmonics in both the line and load side or reduce the specific harmonic frequencies which are undesirable.
2. Description of the Prior Art
One such power converter is shown in U.S. Pat. No. 5,625,545, which is hereby incorporated herein by reference. This converter utilizes a multi-phase input transformer to feed a plurality of series-connected cells. Each series-connected cell includes an incoming rectifier section, which feeds a capacitor bank for storing energy from the rectifier output. The DC voltage from the capacitor is then converted to AC through the utilization of a solid-state inverter. This system provides a reduced harmonic supply in a medium-voltage drive which can be supplied to an AC load such as an AC motor. This topography requires that the energy delivered to the load pass sequentially through a transformer, a rectifier, smoothing capacitors and an inverter. Each of these components increases the cost of the overall system. If the converter uses a simple rectifier, no regeneration (four-quadrant operation) is provided. The use of the DC link also generally requires bulky electrolytic capacitors.
Matrix converters or cycloconverters have been proposed for usage in drive applications. Such matrix converters utilize an array of solid-state switches to sequentially connect the sinusoidally varying AC inputs to respective outputs. Such cycloconverters are limited in their output voltage because they eliminate the DC rectifier. In addition, because the matrix converter switches directly from the AC input to the output lines, the converter produces harmonics at both input and output. Individual solid-state switching devices may have lower voltage ratings than can usually accommodate medium-voltage applications. The cost of medium-voltage-rated solid-state switches can make utilization of matrix converters for medium-voltage applications undesirable. Matrix converters connected directly to the AC line have a limitation in the voltage output, in that the output voltage must relate to the phase-to-phase input voltage. Therefore, when matrix converters are connected to a standard AC line, they generally cannot produce a standard voltage level output that could be used with an existing standard AC motor.
The converter of the invention provides a converter capable of operating as a reduced-harmonic power supply or AC drive. It can provide full four-quadrant operation, permitting power regeneration from the load into the line. In many applications large electrolytic capacitors or bulky reactors may not be required. The output can be either AC or DC, and the voltage and/or frequency may be designed into the equipment to produce the desired output values, including standard motor voltages. Undesired harmonics on the primary and secondary may be significantly reduced. Depending on the application, when desired, the converter can provide power factor improvement.